Multilevel magnetic element

ABSTRACT

The present disclosure concerns a multilevel magnetic element comprising a first tunnel barrier layer between a soft ferromagnetic layer having a magnetization that can be freely aligned and a first hard ferromagnetic layer having a magnetization that is fixed at a first high temperature threshold and freely alignable at a first low temperature threshold. The magnetic element further comprises a second tunnel barrier layer and a second hard ferromagnetic layer having a magnetization that is fixed at a second high temperature threshold and freely alignable at a first low temperature threshold; the soft ferromagnetic layer being comprised between the first and second tunnel barrier layers. The magnetic element disclosed herein allows for writing four distinct levels using only a single current line.

FIELD

The present invention concerns a magnetic element based on a magnetictunnel junction and allowing for writing four different state levels andproviding extended endurance.

BACKGROUND

The development of magnetic random access memory (MRAM) cells with amagnetic tunnel junction has allowed a significant increase in theperformances and operating mode of these MRAMs. Such MRAM cells aredescribed in U.S. Pat. No. 5,640,343. Such MRAM cell typically comprisesa magnetic tunnel junction having a tunneling barrier layer between afirst ferromagnetic layer and a second ferromagnetic layer. The magnetictunnel junction is electrically connected at one end to a first currentline and, to its other end, to a selection CMOS transistor. The MRAMcell may further comprise a second current line disposed orthogonal tothe first current line.

During a write operation of the MRAM cell, the magnetization directionof the first magnetic layer is switched, for example, by applying anexternal magnetic field. In a thermally assisted (TA) write operation,switching the magnetization direction of the first magnetic layer isperformed when the magnetic tunnel junction has been heated to or abovea critical temperature. The magnetic tunnel junction is then cooled downbelow the critical temperature where the first magnetic layermagnetization is “frozen” in the written direction.

During a read operation, the magnetization direction of the secondferromagnetic layer can be compared with the written magnetizationdirection of the first ferromagnetic layer. This is usually performed bypassing a read current through the magnetic tunnel junction such as tomeasure a resistance of the magnetic tunnel junction. A low measuredjunction resistance (or level state “0”) corresponds to themagnetization direction of the second ferromagnetic layer being orientedparallel to the magnetization direction of the first ferromagneticlayer. A high measured junction resistance (or level state “1”)corresponds to the magnetization direction of the second ferromagneticlayer being oriented antiparallel to the magnetization direction of thefirst ferromagnetic layer. The difference between the value of the highand low junction resistance, or the tunnel magnetoresistance, depends onthe material composing the ferromagnetic layers and possibly on heattreatment performed on these ferromagnetic layers.

MRAM cells with a multilevel state write operation has also beenproposed, allowing for writing more than the two level states “0” and“1” as described above. Such a MRAM cell with a multilevel state writeoperation is disclosed in U.S. Pat. No. 6,950,335. Here, themagnetization of the second ferromagnetic layer, or storage layer, canbe oriented in any intermediate direction between the direction paralleland the direction antiparallel to the magnetization direction of thefirst ferromagnetic layer, or reference layer. Orienting themagnetization of the storage layer in the intermediate directions can beachieved by generating magnetic fields with appropriate relativeintensity along the perpendicular directions of the first and secondcurrent line. However, such multilevel MRAM cells require at least twocurrent lines increasing the complexity of the cell.

BRIEF SUMMARY

The present disclosure concerns a magnetic element can comprise a softferromagnetic layer being disposed between a first tunnel barrier layerand a second tunnel barrier layer, a first storage layer, and a secondstorage layer.

The present disclosure further concerns a multilevel magnetic elementcomprising a first tunnel barrier layer between a soft ferromagneticlayer with a magnetization that can be freely aligned and a first hardferromagnetic layer having a fixed magnetization; wherein the magneticelement further comprises a second tunnel barrier layer such that thesoft ferromagnetic layer is between the first and second tunnel barrierlayer, and a second hard ferromagnetic layer having a fixedmagnetization and adjacent to the second tunnel barrier layer.

In an embodiment, the first and second hard ferromagnetic layer can havea magnetization that can be freely aligned at a first predetermined hightemperature threshold and at a second predetermined high temperaturethreshold, respectively, such that, wherein during a write operationsaid magnetic element is heated at the first predetermined hightemperature threshold, and the magnetization of the first storage layeris aligned in a first direction; and said magnetic element is cooled atthe second predetermined high temperature threshold, and themagnetization of second storage layer is aligned in a second direction,such that up to four different state levels can be written in themagnetic element.

In another embodiment, the first high temperature threshold of the firstantiferromagnetic layer can be substantially equal to the second hightemperature threshold of the second antiferromagnetic layer.

In yet another embodiment, the first tunnel barrier layers can have afirst junction resistance-area product that is substantially equal to asecond junction resistance-area product of the second tunnel barrierlayers.

In yet another embodiment, the first predetermined high temperaturethreshold of the first antiferromagnetic layer can be higher than thesecond predetermined high temperature threshold of the secondantiferromagnetic layer.

In yet another embodiment, the first hard ferromagnetic layer and thesecond hard ferromagnetic layer can have different magnetic moments.

In yet another embodiment, said soft ferromagnetic layer can have athickness comprised between 1 nm and 10 nm.

The present disclosure also pertains to a method for writing themagnetic element, comprising:

heating said magnetic element at the first predetermined hightemperature threshold such as to free the magnetization of the firststorage layer;

applying a first write magnetic field such as to align a magnetizationof the first storage layer in accordance with the first magnetic field;

cooling said magnetic element while maintaining the first magnetic fieldto the second predetermined high temperature threshold, such as tofreeze the magnetization of the storage layer in its aligned state andfree the magnetization of the second storage layer;

applying a second write magnetic field such as to align a magnetizationof the second storage layer in accordance with the second magneticfield; and

cooling said magnetic element while maintaining the second magneticfield to the low predetermined high temperature threshold, such as tofreeze the magnetization of the second storage layer in its alignedstate.

In an embodiment, the first write magnetic field can be oriented in adirection opposed to the one of the second write magnetic field, suchthat the magnetization of the first storage layer is aligned in adirection opposed of the magnetization of the second storage layer.

In another embodiment, the first write magnetic field is oriented in thesame direction as the one of the second write magnetic field, such thatthe magnetization of the first storage layer is aligned in the samedirection as the one of the magnetization of the second storage layer.

The present disclosure further concerns a method for writing themagnetic element, comprising:

applying a first read magnetic field such as to align the magnetizationof the sense layer in a first aligned direction in accordance with thefirst read magnetic field;

measuring a first resistance of the magnetic element;

applying a second read magnetic field such as to align the magnetizationof the sense layer in a second aligned direction in accordance with thesecond read magnetic field; and

measuring a second resistance of the magnetic element.

In an embodiment, the first hard ferromagnetic layer and the second hardferromagnetic layer can have different magnetic moments; and the firstread magnetic field can be applied with a zero field value.

The magnetic element disclosed herein allows for writing four distinctlevels using only a single current line. Moreover, the magnetic elementcomprising the first and second tunnel barrier layers allows forreducing the heating current required for heating the magnetic element,resulting in an enhanced endurance of the magnetic element.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with the aid of the descriptionof an embodiment given by way of example and illustrated by the figures,in which:

FIG. 1 shows a magnetic element according to an embodiment;

FIG. 2 illustrates a partial view of the magnetic element of FIG. 1showing the alignment of the magnetization according to an embodiment;and

FIG. 3 represents a configuration comprising the magnetic element and afirst current line, according to an embodiment.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS

A stack, or a magnetic element 2, is schematically represented in FIG. 1according to an embodiment. The magnetic element 2 includes a firstmagnetic tunnel junction portion comprising a first tunnel barrier layer22 having a first junction resistance-area product RA₁, a first hardferromagnetic layer, or first storage layer 21, and a firstantiferromagnetic layer 20. The magnetic element 2 further includes asecond magnetic tunnel junction portion comprising a second tunnelbarrier layer 24 having a second junction resistance-area product RA₂, asecond hard ferromagnetic layer, or second storage layer 25, and asecond antiferromagnetic layer 26.

The first and second storage layers 21, 25 are typically ferromagneticlayers exchange coupled to the first and second antiferromagnetic layers20, 26, respectively, such as to provide an exchange anisotropy thatenables the storage layers 21, 25 to maintain their magnetization whenthe antiferromagnetic layers 20, 26, or the magnetic element 2, are at afirst predetermined low temperature threshold Tw3, and free theirmagnetization when the antiferromagnetic layers 20, 26, or the magneticelement 2, are at a first predetermined high temperature threshold Tw1.The magnetic element 2 further comprises a soft ferromagnetic layer, orsense layer 23, having a magnetization that can be freely aligned anddisposed between the first and second tunnel barrier layer 22, 24. Thearrangement of the magnetic element 2 with the two magnetic tunneljunctions is also called a dual magnetic tunnel junction.

In a first embodiment, the value of the first junction resistance-areaproduct RA₁ of the first tunnel barrier layers 22 is substantially thesame as the value of the second junction resistance-area product RA₂ ofthe second tunnel barrier layers 24. Moreover, the two magnetic tunneljunction portions have the same tunnel magneto resistance TMR, and thefirst and second antiferromagnetic layers 20, 26 have the same firstpredetermined high temperature threshold Tw1. In the present embodiment,the magnetization of the first storage layer 21 is pinned (or fixed) bythe first antiferromagnetic layer 20 at the first low temperaturethreshold Tw3, and is freely alignable at the first high temperaturethreshold Tw1. The magnetization of the second storage layer 25 ispinned (or fixed) by the second antiferromagnetic layer 26 at the firstlow temperature threshold Tw3, and can be freely alignable at a secondhigh temperature threshold Tw2, wherein the second high temperaturethreshold Tw2 is substantially equal to the first high temperaturethreshold Tw1.

Here, a thermally assisted (TA) write operation can comprise (see FIG.3):

heating the magnetic element 2 to the first predetermined hightemperature threshold Tw1, such as to free the magnetization of thestorage layers 21, 25;

applying a first write magnetic field 42 such as to align amagnetization of the first and second storage layers 21, 25 inaccordance with the first write magnetic field 42; and

cooling the magnetic element 2 while maintaining the first writemagnetic field 42 until the magnetic element 2 reaches the predeterminedlow temperature threshold Tw3, such as to freeze the magnetization ofthe storage layers 21, 25 in their aligned state.

Heating the magnetic element 2 is typically performed by applying aheating current 31 in a first current line 4, in electricalcommunication with the magnetic element 2, to the magnetic element 2,possibly via said switching transistor (not shown). Applying the firstwrite magnetic field 42 can be performed by passing a first fieldcurrent 41 in the first current line 4 as illustrated in FIG. 3. FIG. 3represents an exemplary configuration comprising the magnetic element 2,and the first current line 4 for passing the heating current 31 and forpassing the first write current 41 generating the first write magneticfield 42. Other configurations are also possible. For example, the firstwrite current 41 could be passed in a second current line (notrepresented).

A read operation can comprise:

applying a first read magnetic field 53 such as to align themagnetization of the sense layer 23 in a first aligned direction inaccordance with the first read magnetic field 53;

measuring a first resistance R₁ of the magnetic element 2;

applying a second read magnetic field 54 such as to align themagnetization of the sense layer 23 in a second aligned direction inaccordance with the second read magnetic field 54;

measuring a second resistance R₂ of the magnetic element 2; and

determining a difference ΔR between the first resistance value R₁ andthe second resistance value R₂.

Applying the first and second read magnetic field 53, 54 can beperformed by passing, respectively, a first and second read fieldcurrent 51, 52 in the first current line 4 (see FIG. 3). Alternatively,the first and second read magnetic field 53, 54 can be applied bypassing the first and second read field current 51, 52 in the secondcurrent line, respectively. Measuring the first and second resistanceR₁, R₂ can be performed by passing a sense current 32 in the firstcurrent line 4 to the magnetic element 2. The self-referenced readoperation disclosed herein has been disclosed in the not yet publishedU.S. patent application Ser. No. 12/832,472.

An advantage of the magnetic element 2 according to this embodiment isthat the two tunnel barrier layers 22, 24 share the required power toheat the two antiferromagnetic layers 20, 26 to the first predeterminedhigh temperature threshold Tw1. This enable reducing the heating currentrequired for heating the magnetic element 2, or the antiferromagneticlayers 20, 26, at the first predetermined high temperature thresholdTw1, or reducing the corresponding required voltage across each of thefirst and second tunnel barrier layer 22, 24 by a factor ofsubstantially √2 for the same power compared to a conventional stackcomprising only one magnetic tunnel junction. Moreover, there is no lossof read margin compared to the conventional one magnetic tunnel junctionstack with an additional (non-magnetic) heating element. The placementof the second tunnel barrier layer 24 in the vicinity (very close) tothe first tunnel barrier layer 22 ensures that the surface quality ofthe second tunnel barrier layer 24 be very close to that of the firsttunnel barrier layer 22, thereby adding to the overall quality of themagnetic element 2.

In a second embodiment, the first and second tunnel barrier layers 22,24 have different values of the first and second junctionresistance-area product RA₁, RA₂, and the two magnetic tunnel junctionshave possibly different tunnel magneto resistance (TMR) values. Here,the first antiferromagnetic layer 20 has a first predetermined hightemperature threshold Tw1 and the second antiferromagnetic layer 26 hasa second predetermined high temperature threshold Tw2.

In this configuration, four distinct levels can be written in themagnetic element 2, and sensed using the read operation described above,as outlined in Table 1. Let's assume that the first predetermined hightemperature threshold Tw1 of the first antiferromagnetic layer 20 ishigher than the second predetermined high temperature threshold Tw2 ofthe second antiferromagnetic layer 24, such at Tw2, the magnetization ofthe first storage layer 21 is frozen and the magnetization of the secondstorage layer 25 can be freely aligned. Then the write operation cancomprise:

heating the first antiferromagnetic layer 20 to the first predeterminedhigh temperature threshold Tw1, such as to free the magnetization of thefirst storage layer 21;

applying the first write magnetic field 42 such as to align amagnetization of the first storage layer 21 in accordance with the firstmagnetic field 42;

cooling the first antiferromagnetic layer 20 while maintaining the firstmagnetic field 42 until the layer 20 reach the second predetermined hightemperature threshold Tw2, such as to freeze the magnetization of thestorage layer 21 in its aligned state and free the magnetization of thesecond storage layer 25;

applying a second write magnetic field 44 such as to align amagnetization of the second storage layer 25 in accordance with thesecond magnetic field 44; and

cooling the second antiferromagnetic layer 26 while maintaining thesecond magnetic field until the layer 26 reach the low predeterminedhigh temperature threshold Tw3, such as to freeze the magnetization ofthe second storage layer 25 in its aligned state.

The first write magnetic field 42 can be oriented in a direction opposedto the one of the second write magnetic field 44 such that themagnetization of the first storage layer 21 is aligned in a directionopposed of the magnetization of the second storage layer 25. In thiscase, the write operation described above can be used to write fourdistinct state levels in the magnetic element 2. Applying the secondwrite magnetic field 44 can be performed by passing a second fieldcurrent 43 in the first current line 4 (FIG. 3)

TABLE 1 state1 state2 state3 state4 R with ref to left ← Rmax1 + Rmax2Rmax1 + Rmin2 Rmin1 + Rmax2 Rmin1 + Rmin2 R with ref to right → Rmin1 +Rmin2 Rmin1 + Rmax2 Rmax1 + Rmin2 Rmax1 + Rmax2 Write sequence Heatabove Tw1 & Heat above Tw1 & Heat above Tw1 & Heat above Tw1 & Tw2 Tw2with H → then Tw2 with H → Tw2 with H ← with H ← then cool with coolwith H → and cool to and cool to H ← Tw1 > T > Tw2 Tw1 > T > Tw2 reverseH ← and reverse H → and complete cooling complete coolingor in the second current line.

Table 1 summarizes the write operation according to the embodiment wherefour distinct state levels in the magnetic element 2. In FIG. 2, thecorresponding alignment of the magnetization of the storage layers 21,25 and the sense layer 23 is represented by the arrows. FIG. 2illustrates a partial view of the magnetic element of FIG. 1 showing thetwo storage layers 21, 25, the two tunnel barrier layers 22, 24 and thesense layer 23.

More particularly, in a state level “state1” (FIG. 2 (a)), theorientation of the first and second write magnetic field is identicaland the aligned magnetization direction of the first storage layer 21 isidentical to the aligned magnetization direction of the second storagelayer 25. During the read operation, the first read magnetic fieldoriented in the same direction as the aligned magnetization direction ofthe first and second storage layers 21, 25 yields large measured firstand second resistance values (R_(max1)+R_(max2)). The second readmagnetic field yields small measured first and second resistance values(R_(min1)+R_(min2)).

In a state levels “state2” and “state3” (FIG. 2 (b) and (c)), theorientation of the first write magnetic field is opposed to the one ofthe second write magnetic field. In the example of Table 1 and FIG. 2,the magnetization of the first and second storage layers 21, 25 isaligned such that the values of the measured first and second resistance(R_(max1)+R_(min2)) and (R_(min1)+R_(max2)) for the state level“state2”, is distinct than the values of the measured first and secondresistance (R_(max1)+R_(max2)) and (R_(max1+R) _(min2)) for the statelevel “state3”.

In a state level “state4” (FIG. 2 (d)), the orientation of the firstwrite magnetic field is opposed is identical but opposed that the oneused in state level “state1”, and the magnetization of the first andsecond storage layers 21, 25 is aligned such that the values of themeasured first and second resistance (R_(min1)+R_(min2)) when applyingthe first read magnetic field are small, and the values of the measuredfirst and second resistance (R_(max1)+R_(max2)) when applying the firstread magnetic field are large.

These four state levels allow for the coding of two bits per cell (orper magnetic element 2) with the use of the single current line.Moreover, the magnetic element 2 has an enhanced endurance due to thedual magnetic tunnel junction arrangement as described above.

In an embodiment, the first antiferromagnetic layer 20 can be made fromIrMn, the first storage layer 21 can be made from NiFe₂/CoFeB₂, thefirst tunnel barrier layer 22 can be made from MgO with a first junctionresistance-area product (RA₁) of about 20, and the sense layer 23 can bemade from CoFeB₂. The sense layer 23 can have a thickness comprisedbetween 1 nm and 10 nm. The second tunnel barrier layer 24 can be madefrom MgO with a first junction resistance-area product (RA₂) of about40, and the second storage layer 25 can be made from NiFe₂/CoFeB₂, andthe second antiferromagnetic layer 26 from FeMn. Using MgO for formingthe tunnel barrier layers 22, 24 with MgO can advantageous produceextremely flat tunnel barrier layers 22, 24. This is a desirable qualityfor high performance magnetic tunnel junctions used in TAS-MRAMapplications.

Table 2 reports the resistance level values determined in the case ofthe magnetic element 2 having a diameter of about 0.15 μm and with thelayers 20, 21, 22, 23, 24, 25, 26 having the composition describedabove.

TABLE 2 first second resistance, resistance, Difference magnetizationmagnetization between first of sense layer of sense layer and secondState level pointed left pointed right resistances State 1 7922 33954527 State 2 5093 6791 −1698 State 3 6791 5093 1698 State 4 3395 7922−4527

Many alternative embodiments are possible including the use of apartially compensated SAF reference/sense layer (layer between the twotunnel barriers 22, 24).

In another embodiment, the two ferromagnetic storage layers 21, 25 havedifferent magnetic moments. This results in a well defined orientationfor the sense layer 23 in the absence of the write magnetic field 42,44.

Here, the read operation can comprise measuring the first resistance R₁of the magnetic element 2 without applying the first read magnetic field53 (or applying the first read magnetic field 53 with a zero fieldvalue), followed by measuring the second resistance R₂ of the magneticelement 2 when applying the second read magnetic field 54 having asingle magnetic field direction.

The four state levels can be determined by measuring the initialresistance value and the change in resistance, as represented in Table3. This allows for reducing the read operation time and powerconsumption for performing the read operation. Table 3 describe the readand write sequence according to this embodiment.

TABLE 3 state1 state2 state3 state4 R in H = 0 Rmax1 + Rmax2 Rmin1 +Rmax2 Rmin1 + Rmax2 Rmin1 + Rmin2 R with ref to right → Rmin1 + Rmin2Rmin1 + Rmax2 Rmax1 + Rmin2 Rmax1 + Rmax2 Write sequence Heat above Tw1& Heat above Tw1 & Heat above Tw1 & Heat above Tw1 & Tw2 Tw2 with H →then Tw2 with H → Tw2 with H ← with H ← then cool with cool with H → andcool to and cool to H ← Tw1 > T > Tw2 Tw1 > T > Tw2 reverse H ← andreverse H → and complete cooling complete cooling

Table 4 reports the resistance level values for the stack having adiameter of about 0.15 μm and the different layers having thecompositions as described above.

In another embodiment, the read operation can comprise measuring aresistance of the magnetic element 2 when applying a read magnetic fieldhaving a single magnetic field direction, followed by measuring aresistance of the magnetic element 2 without applying the read magneticfield.

TABLE 4 first second Difference resistance, no resistance, between firstmagnetic field magnetic field and second State level applied (H = 0)applied resistances State 1 7922 3395 4527 State 2 6791 6791 0 State 36791 5093 1698 State 4 7922 7922 0

The configuration of the magnetic element 2 according to the embodimentsis advantageous compared to conventional TAS MRAM stacks. Indeed, thedual magnetic tunnel junction configuration described herein comprisingtwo tunnel barrier layers 22, 24 separated by a thin ferromagnetic senselayer 23, enables the two tunnel barrier layers 22, 24 to be formed withhigh quality.

The magnetic element 2 with the dual magnetic tunnel junction reducesthe voltage drop on each tunnel barrier layer 22, 24 during the writeoperation (total voltage shared between the two tunnel barrier layers22, 24). The use of two different antiferromagnetic layers 20, 26, eachhaving a different predetermined high temperature threshold Tw1, Tw2,and each being coupled to the first and second storage layer 21, 25,allows for having up to four different resistance state levels per cell(or magnetic element 2) thus increasing the storage capacity from 1bit/cell to 2 bits/cell. Note that the magnetic element 2 disclosedherein is distinct from the dual MTJ for TAS-MRAM described inpublication J. App. Phys. 99 08N901 (2006).

REFERENCE NUMBERS AND SYMBOLS

-   2 magnetic element, stack-   20 first antiferromagnetic layer-   21 first storage layer-   22 first tunnel barrier layer-   23 reference or sense layer-   24 second tunnel barrier-   25 second storage layer-   26 second antiferromagnetic layer-   31 heating current-   32 sense current-   4 first current line-   41 first field current-   42 first write magnetic field-   43 second write magnetic field-   44 second write magnetic field-   51 first read field current-   52 second read field current-   53 first read magnetic field-   54 second read magnetic field-   RA₁ first junction resistance-area product-   RA₂ second junction resistance-area product-   R₁ first resistance-   R₂ second resistance-   ΔR difference between the first and second resistances-   TMR tunnel magneto resistance-   Tw1 first high temperature threshold-   Tw2 second high temperature threshold-   Tw3 first low temperature threshold

1. A multilevel magnetic element comprising a first tunnel barrier layerbetween a soft ferromagnetic layer having a magnetization that can befreely aligned and a first hard ferromagnetic layer having amagnetization that is fixed at a first high temperature threshold andfreely alignable at a first low temperature threshold; wherein themagnetic element further comprises a second tunnel barrier layer and asecond hard ferromagnetic layer having a magnetization that is fixed ata second high temperature threshold and freely alignable at a first lowtemperature threshold; the soft ferromagnetic layer being comprisedbetween the first and second tunnel barrier layers.
 2. The magneticelement according to claim 1, wherein the first high temperaturethreshold of the first antiferromagnetic layer is substantially equal tothe second high temperature threshold of the second antiferromagneticlayer.
 3. The magnetic element according to claim 1, wherein the firsttunnel barrier layers has a first junction resistance-area product thatis substantially equal to a second junction resistance-area product ofthe second tunnel barrier layers.
 4. The magnetic element according toclaim 1, wherein the first predetermined high temperature threshold ofthe first antiferromagnetic layer is higher than the secondpredetermined high temperature threshold of the second antiferromagneticlayer.
 5. The magnetic element according to claim 1, wherein the firsthard ferromagnetic layer and the second hard ferromagnetic layer havedifferent magnetic moments.
 6. The magnetic element according to claim1, wherein said soft ferromagnetic layer has a thickness comprisedbetween 1 nm and 10 nm.
 7. A magnetic element comprising: a first hardferromagnetic layer, a soft ferromagnetic layer, a first tunnel barrierlayer, a second hard ferromagnetic layer, and a second tunnel barrierlayer; the soft ferromagnetic layer being between the first tunnelbarrier layer and the second tunnel barrier layer.
 8. A method forwriting to a magnetic element, the magnetic element comprising: a softferromagnetic layer having a magnetization that can be freely aligned; afirst hard ferromagnetic layer having a magnetization that is fixed at afirst high temperature threshold and freely alignable at a first lowtemperature threshold; a first tunnel barrier layer between the softferromagnetic layer and the first hard ferromagnetic layer; a secondhard ferromagnetic layer having a magnetization that is fixed at asecond high temperature threshold and freely alignable at a first lowtemperature threshold; and a second tunnel barrier layer; the softferromagnetic layer being comprised between the first and second tunnelbarrier layers; the method comprising: heating said magnetic element tothe first high temperature threshold; applying a first write magneticfield such as to align the magnetization of the first storage layer andthe magnetization of the second storage layer in accordance with thefirst magnetic field; cooling said magnetic element to the lowtemperature threshold such as to freeze the magnetization of the firstand second storage layers in their aligned state.
 9. Method according toclaim 8, wherein the first predetermined high temperature threshold ishigher than the second predetermined high temperature threshold; themethod further comprising: cooling said magnetic element to the secondhigh temperature threshold; and applying a second write magnetic fieldsuch as to align the magnetization of the second storage layer inaccordance with the second magnetic field; such that up to four distinctstate levels can be written in the magnetic element.
 10. Methodaccording to claim 9, wherein the first write magnetic field is orientedin a direction opposed to the one of the second write magnetic field,such that the magnetization of the first storage layer is aligned in adirection opposed of the magnetization of the second storage layer. 11.Method according to claim 9, wherein the first write magnetic field isoriented in the same direction as the one of the second write magneticfield, such that the magnetization of the first storage layer is alignedin the same direction as the one of the magnetization of the secondstorage layer.
 12. A method for reading the magnetic element, themagnetic element comprising: a soft ferromagnetic layer having amagnetization that can be freely aligned; a first hard ferromagneticlayer having a magnetization that is fixed at a first high temperaturethreshold and freely alignable at a first low temperature threshold; afirst tunnel barrier layer between the soft ferromagnetic layer and thefirst hard ferromagnetic layer; a second hard ferromagnetic layer havinga magnetization that is fixed at a second high temperature threshold andfreely alignable at a first low temperature threshold; and a secondtunnel barrier layer; the soft ferromagnetic layer being comprisedbetween the first and second tunnel barrier layers; the methodcomprising: applying a first read magnetic field such as to align themagnetization of the sense layer in a first aligned direction inaccordance with the first read magnetic field; measuring a firstresistance of the magnetic element; applying a second read magneticfield such as to align the magnetization of the sense layer in a secondaligned direction in accordance with the second read magnetic field; andmeasuring a second resistance of the magnetic element.
 13. Methodaccording to claim 12, wherein the first hard ferromagnetic layer andthe second hard ferromagnetic layer have different magnetic moments; andwherein the first read magnetic field is applied with a zero fieldvalue.